Last January, a team of researchers searching for the coronavirus in New York City’s wastewater spotted something strange in their samples. The viral fragments they found had a unique constellation of mutations that had never been reported before in human patients — a potential sign of a new, previously undetected variant.

For the past year, these oddball sequences, or what the scientists call “cryptic lineages,” have continued to pop up in the city’s wastewater.

There is no evidence that the lineages, which have been circulating for at least a year without overtaking Delta or Omicron, pose an elevated health risk to humans. But the researchers, whose findings were published in Nature Communications on Thursday, still have no idea where they came from.